Proper cellular function and differentiation depends on intrinsic signals and extracellular environmental cues. These signals and cues vary over time and location in a developing organism (i.e., during embryogenesis), and remain important in developing and differentiating cells during post-natal growth and in a mature adult organism. Thus, in a general sense, the interplay of the dynamically changing set of intracellular dynamics (such as manifested by intrinsic chemical signaling and control of gene expression) and environmental influences (such as signals from adjacent cells) determine cellular activity. The cellular activity so determined is known to include cell migration, cell differentiation, and the manner a cell interacts with surrounding cells.
The use of stem cells and stem-cell-like cells of various types for cell replacement therapies, and for other cell-introduction-based therapies, is being actively pursued by a number of researchers. Embryonic stems cells from a blastocyst stage are frequently touted for their pluripotency—that is, their ability to differentiate into all cell types of the developing organism. Later-stage embryonic stem cells, and certain cells from generative areas of an adult organism, are identified as more specialized, multipotent stem cells. These cells include cells that are able to give rise to a succession of a more limited subset of mature end-stage differentiated cells of particular types or categories, such as hematopoietic, mesenchymal, or neuroectodermal end-stage differentiated cells. For example, a multipotent neural stem cell may give rise to one or more neuron cell types (i.e., cholinergic neuron, dopaminergic neuron, GABAergic neurons), which includes their specific cell classes (i.e., a basket cell or a chandelier cell for GABAergic neurons), and to non-neuron glial cells, such as astrocytes and dendrocytes.
Further along the path of differentiation are cells derived from multipotent stem cells. For example, derivatives of a localized, non-migrating neuroectodermal type stem cell may migrate but, compared to their multipotent parent, have more limited abilities to self-renew and to differentiate (See Stem Cell Biology, Marshak, Gardner & Gottlieb, Cold Spring Harbor Laboratory Press, 2001, particularly Chapter 18, p. 407). Some of these cells are referred to a neuron-restricted precursors (“NRPs”), based on their ability, under appropriate conditions, to differentiate into neurons. There is evidence that these NRPs have different subclasses, although this may reflect different characteristics of localized multipotent stem cells (Stem Cell Biology, Marshak et al., pp 418-419).
One advantage of use of mulitpotent and more committed cells further along in differentiation, compared to pluripotent embryonic stem cells, is the reduced possibility that some cells introduced into an organism from such source will form a tumor (Stem Cell Biology, Marshak et al., p. 407). However, a disadvantage of cells such as cell types developed from multipotent stem cells, for instance, embryonic progenitor cells, is that they are not amenable to ongoing cell culture. For instance, embryonic neural progenitor cells, which are able to differentiate into neurons and astrocytes, are reported to survive only one to two months in a cell culture.
There is a need in the art to improve the compositions, methods and systems that provide biased and/or differentiated cells from stem cells or stem-cell-like cells. More particularly, a need exists to obtain a higher percentage of desired cells from a pre-implantation cell culture, such as starting from multipotent stem cells and obtaining a higher percentage of cells committed to differentiate to a specified type of functional nerve cell, such as cholinergic neurons or inner ear hair cells. The present invention addresses these needs.